A flexible copper clad laminate (FCCL) is mainly used as a substrate for a flexible printed circuit board (FPCB), and also used for a surface heating element electromagnetic wave shielding material, a flat cable, a packaging material, and the like. As an electronic device employing a PCB has recently gradually become small-sized, highly densified, and highly effective, the use of a FCCL has been further increased.
The FCCL includes a polyimide layer and a copper clad layer. Such a FCCL is prepared by laminating a copper clad layer onto a polyimide film, or prepared by laminating a polyimide layer onto a copper clad layer. In other words, the FCCL is prepared by an adhesive method, or by a cast method.
The FCCL prepared by the adhesive method is prepared by attaching a copper clad layer on a polyimide film via an epoxy adhesive. In the case of such a FCCL, the adhesive film is weak to heat, and under high temperature.high pressure environment, copper ions of the copper clad layer are moved into the adhesive film layer, thereby causing a short between circuit wirings. In other words, properties such as heat resistance, chemical resistance, flame retardancy and electrical characteristics of a final FCCL depend on the characteristics of the used adhesive film. Accordingly, the polyimide resin cannot sufficiently exhibit its natural characteristics such as high flexibility and heat resistance.
Meanwhile, unlike the FCCL prepared by the adhesive method, the FCCL prepared by the cast method is prepared by applying polyimide varnish on a copper clad layer, carrying out heat treatment under an appropriate condition, and then forming a polyimide resin layer on the copper clad layer, without an adhesive.
However, in the FCCL prepared by the cast method, curling occurs due to a difference in thermal expansion coefficients between the polyimide layer and the copper clad. Thus, in order to prevent the curling, various FCCL preparation methods have been developed.
Especially, a method for preparing a FCCL by varying a thermal expansion coefficient of a polyimide resin layer with respect to a copper clad has been widely used. In a FCCL prepared by this method, it is possible to prevent the curling due to improvement of heat resistance or flexibility, but there is a problem in that the adhesive strength of the polyimide resin layer is decreased. For this reason, it was difficult to adhere the FCCL to a bonding sheet (bonding prepreg) used for preparation of a multilayer flexible circuit board, thereby declining the applicability of the FCCL in the multilayer flexible PCB.
Accordingly, in the preparation of a FCCL, it is required to develop a novel polyimide resin with improved adhesive strength as well as improved various performances such as heat resistance and flexibility.